Fix MinGW build broken by recent update from Gnulib

Gnulib moved the prototypes of 'sig2str' and 'str2sig' from
sig2str.h to signal.h, but MinGW does not use the Gnulib
signal.h header, and doesn't have these functions in its
own signal.h.
* lib/sig2str.h (SIG2STR_MAX, sig2str, str2sig): Define and
prototype if not already defined.

lib/sig2str.h

@@ -19,6 +19,21 @@
 
 #include <signal.h>
 
+/* Maximum size of a signal name returned by sig2str(), including the
+   terminating NUL byte.  */
+#ifndef SIG2STR_MAX
+/* The longest one: "RTMAX", then "+" or "-", then up to 10 digits, then NUL.
+   Add + 2 as a reserve for the future.  */
+# define SIG2STR_MAX (5 + 1 + 10 + 1 + 2)
+#endif
+
+#ifndef HAVE_SIG2STR
+int sig2str (int, char *);
+#endif
+#ifndef HAVE_STR2SIG
+int str2sig (char const *, int *);
+#endif
+
 /* An upper bound on signal numbers allowed by the system.  */
 
 #if defined _sys_nsig